The broad long-term objective is to develop a human immunodeficiency virus type 1 (HIV-1) recombinant gpl20 prime, peptide boost vaccination strategy using an immunogen, adjuvant and injection schedule, which would be acceptable for human use. The specific aim of the present project is to study the neutralizing and enhancing responses, assayed with laboratory and primary isolates of homologous and heterologous HIV-1 clade B isolates, in rhesus monkeys, rabbits and rats primed with recombinant HIV-1 SF2gpl20 immune stimulating complexes (ISCOM) formulations and boosted with synthetic peptides. The research design and methods will be as follows: A sensitive assay will be developed for monitoring neutralizing and enhancing antibody against the homologous HIV-1 SF2 and heterologous primary clade B viruses (HIV-1 DH12, HIV-1 HAN2, for which stocks titrated in chimps exist) in already existing sera of successfully prime-boost vaccinated rhesus-monkeys. Non-phytohemagglutinin (PHA) stimulated peripheral blood mononuclear cells (PBMC), as well as enriched purified T4, CD3+, CD4+, HLA-DR+ non-PHA stimulated cells, will be evaluated as host cells in the neutralization assay. Enhancement of the monkey sera against primary clade B viruses (see above) will be monitored in enriched human macrophage cultures. In primed and boosted rabbits and rats, the effect of the following parameters on the production of neutralizing and/or enhancing antibodies against laboratory and primary HIV-1 clade B viruses will be studied: priming with HIV-2 SF2 gpl20 coupled to ISCOM either via their protein backbone or their carbohydrates; the effect of two prime boost regimens, prime with recombinant immunogen and boost with peptides versus prime with peptides and boost with recombinant immunogen; boosting with mimotopes which have shown high affinity to the human monoclonal antibody (mAb) immunoglogulin G (IgG) 1 bl2, which is known to neutralize primary HIV-1 isolates; the use of single copy 15 mer V2, V3 and CD4 peptide to boost versus shorter, overlapping peptides, presented as a multiple antigenic peptide. The prime-boost regimen, which induces in rabbits and rats the broadest cross-neutralization response and the lowest enhancement against the various primary HIV-1 clade B viruses, will be selected to go forward to challenge studies in rhesus monkeys with chimeric simian/human immunodeficiency virus (SHIV)-DH12 or SHIV HAN2. Ultimately, the most successful approach will be tested later on in chimpanzees.